The present invention relates to a sealed storage cell having a capacity greater than 10 Ah and comprising a practically cylindrical container, an electrochemical stack, and two opposite ends with terminals of opposite polarity, at least one end containing aluminum. The storage cell is used in particular to power electrical or hybrid vehicles, typically having a capacity of 30 Ah to 50 Ah. The invention relates more particularly to sealed storage cells in which the electrochemical couple is one that operates in the presence of a non-aqueous electrolyte such as the lithium-ion couple.
High-capacity sealed lithium-ion storage cells used in the automobile industry for the main or back-up propulsion of electric vehicles usually have a stainless steel or nickel-plated steel container and a pure aluminum positive terminal.
One advantage of using aluminum is its low density, which increases the capacity per unit mass of a cell in which it is used for the cover and/or the container. On the other hand, pure aluminum is mechanically weaker than nickel-plated steel or stainless steel. This is why, if the cover at the positive end of a cell is made of aluminum, it is difficult to design a system for connecting an external connection bar to the cell which is compact, reliable and stable in time and which does not generate too high an electrical resistance as it ages, especially if the cell has high capacity and the connections must be thick in order to carry high currents. To be effective and stable in time, aluminum screw fasteners necessitate the use of a screwthread occupying a relatively large volume, to achieve a good grip in the material. However, the benefit of using aluminum, in terms of capacity per unit volume, is then lost because of the volume occupied by the connections. As a result of this the improvement in terms of capacity per unit volume is minor, despite the increase in capacity per unit mass.
European Patent Application EP-A2-0 771 040 describes a sealed cylindrical storage cell comprising an electrochemical stack inside a stainless steel container. It can be used to power electric vehicles. The steel cover incorporates an aluminum terminal and can include a system for evacuating gas from inside the cell in the event of a malfunction. This system takes the form of a relief valve at an off-center position on the cover. The cover also includes an electrolyte filler orifice at an off-center position symmetrical to that of the valve. A cell of this kind has drawbacks. For one thing, the position of the relief valve means that when a connecting bar is fitted, for example when the cell is used to power a hybrid electric vehicle, the bar must be positioned so that it does not impede the operation of the valve. Also, the volume of the positive end or terminal is very large, because ultra-pure aluminum is a particularly soft material, and this represents an unacceptable loss in terms of the capacity per unit mass and the capacity per unit volume of the cell. Finally, the fact that the container is made of steel and the positive terminal is made of aluminum makes it essential to isolate these two components electrolytically, by means of a gasket, which increases costs and introduces the risk of leaks. There is already a risk of leaks at the negative terminal, which is made of copper. It is therefore difficult to accept doubling the risk of leaks from the cell by using gaskets at both ends.
Accordingly, one object of the invention is to provide a sealed high-capacity storage cell whose container is practically cylindrical, which has two terminals at the two ends of the practically cylindrical container, and in which one of the two terminals includes a current collection arrangement made of aluminum that is compact, reliable, and efficient throughout the service life of the cell.
The present invention provides a sealed storage cell, of capacity greater than 10 Ah, including a practically cylindrical container, an electrochemical stack, and two opposite ends providing terminals of opposite polarities, one of the ends comprising aluminum, wherein:
said one end comprises an aluminum cover, said aluminum cover is adapted to be brought into electrical contact with an electrical connecting part external to the cell by clamping means which are at least in part on the side of said cover inside the cell and which cooperate with assembly means external to the cell, and said clamping means are made from a material selected from the group comprising stainless steel, nickel-plated steel, copper, and brass; and
said cover of said cell is sealed by a metal sealing cap on the side of the cover inside the cell.
The use of the material chosen for the clamping means has the advantage that a high clamping torque can be used for mounting the end of the cell and for making a connection to an external electrical connecting part.
In one embodiment, the container of the cell is made of aluminum. The advantage of aluminum over the stainless steel used in the prior art is improved capacity per unit mass. Another advantage compared to the prior art is an improved seal, as there is no need for a gasket at the end of the cell having the aluminum cover to prevent problems caused by electrolytic corrosion.
In one embodiment of the cell of the invention, the clamping means comprise a screwthread. This advantageously provides a screwed connection to an external connecting part that is highly reliable and highly stable, particularly in the case of a high-capacity cell. In a first variant, the clamping means comprise a part bearing on the face of the cover inside the cell and having a screwthread. For example, the clamping means comprise a nut or a pin. In a second variant, the clamping means comprise a bush.
The clamping means are preferably associated with a rotation-preventing system. This facilitates assembling the cell.
In one embodiment the cover includes openings enabling gas to escape from the cell.
In a different embodiment the metal sealing cap includes at least one weakened area adapted to rupture if the pressure inside the cell increases to an internal pressure value greater than or equal to a pressure P1.
In a preferred embodiment the metal sealing cap is pressed against said clamping means which can then be adapted to prevent deformation of a part of said metal sealing cap, preferably a central part thereof, in the event of a pressure rise inside the cell up to a value of the internal pressure greater than or equal to the pressure P1. This embodiment has the advantage of contributing to ensuring that tearing of the weakened areas of said cap occurs cleanly at the required value, reducing the margin for error.
In a different embodiment the cell of the invention includes a switch having a sensor installed in a housing via an opening in the cover so that the sensor faces the metal sealing cap, the switch further having two terminals for making an electrical connection to an electronic disconnector device adapted to interrupt the flow of current in the cell in the event of deformation of said metal sealing cap due to the pressure inside the cell rising to a value greater than or equal to a pressure P2, the pressure P2 being less than or equal to the pressure P1. This has an advantage in terms of safety, by enabling the source of the malfunction to be cut off as soon as possible before the safety device comprising the weakened areas of the metal sealing cap is tripped. A portion of said metal sealing cap is preferably adapted to be turned inside out by the pressure inside the cell rising to a value greater than or equal to the pressure P2, so as to be in electrical contact with the switch. The contact is then a clean contact.
The invention relates more particularly to sealed storage cells in which the electrochemical couple is one that operates in the presence of a non-aqueous electrolyte such as the lithium-ion couple. Such cells are generally used to power electric and hybrid vehicles.